MicroRNAs play critical roles in normal development and disease pathogenesis. There is a global down-regulation of microRNAs in cancer suggesting that most microRNAs may act like tumor suppressors. However, a small number of microRNAs are up-regulated in cancer, some of which have been shown to be oncogenic. The biological roles of only a small fraction of identified microRNAs have been elucidated to date. We and others have found that the tumor suppressor microRNA miR-34a inhibits proliferation and apoptosis in a colon cancer cell line (HCT116) that expresses wild-type p53. Interestingly, we also found that miR-34a has no effect on proliferation or viability in an isogenic HCT116 cells that are p53 null suggesting that the tumor suppressor function of miR-34a is controlled by p53. The context-dependent function may not be restricted to miR-34a, and other microRNAs may also exhibit cell-type or tissue-specific functions. Indeed, in FY11, we have found that regulation of proliferation by miR-24 and miR-191 is cell-type specific. Moreover, recent studies have shown that genetic ablation of the miR-17-92 cluster in mice suppresses tumorigenicity only in the context of Myc-driven B-cell lymphomas. These findings suggest that the function of microRNAs such as miR-34a, miR-24, miR-191 and miR-17-92 cluster may be dependent on the expression of oncogenes and/or tumor suppressor genes and highlight the need to understand the molecular basis of their context dependent function. We hypothesize that genetic and epigenetic changes that occur during tumorigenesis alter the balance between pro- and anti-proliferative targets regulated by miR-24 and miR-191, leading to their context-dependent function. We plan to test our hypothesis by identifying the genes and pathways regulated by miR-24 and miR-191 in HeLa and A549 cells. We have chosen these cell lines to initiate this study since we know that miR-24 and miR-191 have opposite effects on the proliferation in HeLa and A549 cells. To further gain mechanistic insights on the cell-type specific function of these microRNAs we will identify the mRNAs directly regulated by miR-24 and miR-191 in HeLa and A549 cells. We will also determine the influence of changes in target mRNA copy number, alternative polyadenylation and 3UTR mutations on the function of miR-24 and miR-191. Finally, we will investigate the influence of these miR-24 and miR-191 on the proliferation and viability of a panel of lung, breast and colorectal cancer cell lines. Parallel to this, in FY12, we have performed a miRNA over-expression screen in HCT116 p53 wild-type and isogenic HCT116 p53 knockout cells using a miRNA mimic over-expression library (Dharmacon). We have found that many miRNAs require p53 to suppress cell growth and proliferation. We are currently investigating whether these miRNAs exhibit context-dependent function in a panel of cell lines that are p53-proficient or p53-null.These studies will also enable us to determine whether the function of these microRNAs depends on the tissue from which the cancer originates.